1. Field of the Invention
This invention relates to a coated breaker for use with acid fracturing gels used in treating subterranean formations.
2. Description of the Prior Art
Subterranean formations of oil and gas wells are often treated by hydraulically fracturing the formations to increase production of the oil or gas therein. Fracturing of these formations is accomplished by pumping fluids into the bore holes of the oil or gas wells under high pressure so that cracks or fissures are opened in the surrounding formation. Typically, the fracturing fluid is a polymer which has been gelled to increase its viscosity. The fluid's viscosity is proportionally related to the created fracture geometry and fracture width so that the more viscous fluids will produce longer and wider fractures. After the fracturing fluid is injected into the formation to produce the fracture, the viscosity of the fluid is reduced by means of "gel breakers" which break down the gelled fluid so that it can be easily pumped and removed from the well.
In certain formations, aqueous acid solutions can be used to improve the permeability of the formation thereby increasing production. These acids are often combined with the polymer gels used in fracturing to provide an acid fracturing fluid. One of the benefits of combining the aqueous acid solutions with gelled fracturing fluids is that the gelled fluid inhibits or retards the reaction of the acid with the formation. This is beneficial in that the acid would otherwise react too quickly, depleting the acid with very little penetration of the formation. Once in place, the viscosity of the fluid is reduced so that the acid is released to react, thus dissolving the faces of the newly formed fractures and improving the permeability of the producing strata.
Crosslinked polymer gels have been particularly useful with these aqueous acid solutions. The crosslinked gels are able to withstand the high temperature conditions commonly found in deeper oil and gas wells with little reduction in viscosity. The crosslinked polymer gels also exhibit an improved ability in reducing the reaction rate of the acid solution. Organometallic compounds are often used as a crosslinking agent in these polymer gels. It has been found that gels crosslinked with zirconium and titanium compounds can be treated with certain gel breaking substances, such as fluoride, phosphate or sulphate anions, to break the linkages of the crosslinked polymer fluid, thus reducing the viscosity of the gel.
Due to the depth of most wells, gel breakers dispersed within these crosslinked fluids prior to pumping would react too quickly, causing the fluid to degrade before placement of the fluid within the producing area of the formation. As a result, gel breakers have been added after the fluid is in place. This, however, requires additional time and labor. It is also difficult to ensure that the subsequently added breakers become adequately dispersed throughout the fluid so that the fluid becomes fully degraded.
Retarded gel breakers have been developed which can be dispersed throughout the fluid prior to injecting it into the well. The delayed reaction of these retarded breakers allows the gelled fracture acidizing fluid to be pumped into the well with little effect on its viscosity. Once the fluid is in place, the viscosity of the fluid is gradually reduced. This releases the acid and facilitates the subsequent removal of the fluid from the formation. These retarded gel breakers are typically formed by coating or encapsulating particles of certain breaker materials so that the release of the breaker is delayed. There are problems with these gel breakers however. Often solvents used in the coating of these gel breakers leave the resulting particles sticky and moist so that the particles are difficult to handle and disperse within the fluid.